Ceric oxide with new morphological characteristics and method for obtaining same

ABSTRACT

A ceric oxide having a specific surface of at least 15 m 2  /g, measured after calcination at a temperature of between 800° and 900° C.

This application is a divisional of application Ser. No. 07/213,189filed Jun. 29, 1988.

The object of the present invention is a ceric oxide with newmorphological characteristics. The invention also relates to one of themethods for obtaining said oxide.

In the following description of the invention, "Specific surface" meansthe specific B.E.T. surface determined by nitrogen adsorption inaccordance with the ASTM D 3663-78 standard established by theBRUNAUER-EMMETT-TELLER method described in "The Journal of AmericanSociety, 60, 309 (1938)".

It is known that ceric oxide can be used as a catalyst or as a catalystsupport. For example, the works of Paul MERIDEAU et al can be citedrelative to the synthesis of methanol from CO+H₂ on platinum catalystsdeposited on the ceric oxide. (C.R. Acad. Sc. Paris Vol. 297--SeriesII-471--1983).

It is also well known that a catalyst is generally more effective whenthe contact surface between the catalyst and the reagents is larger. Forthis purpose, it is necessary for the catalyst to be maintained in themost divided state possible, that is, that the solid particles whichcompose it be as small and individualized as possible. The fundamentalrole of the support, therefore, is to maintain the catalyst particles orcrystallites in contact with the reagents, in the most divided statepossible.

During the extended use of a catalyst support, a decrease in thespecific surface occurs due to the coalescence of the very finemicropores. During this coalescence, part of the catalyst is surroundedby the body of the support and can no longer be in contact with thereagents.

Until now, the majority of ceric oxides prepared had a specific surfacewhich decreased rapidly for working temperatures greater than 500° C.Thus, from an ammonium cerinitrate, R. ALVERO et al (J. Chem. Soc.Dalton Trans 1984, 87) obtained a ceric oxide having a specific surfaceof 29m² /g after calcination at a temperature of 600° C.

In addition, FR-A 2,559,754 describes a ceric oxide with a specificsurface of at least 85±5 m² /g obtained after calcination between 350and 450° C. and, preferably, between 100 and 130 m² /g after calcinationbetween 400 and 450° C. Said oxide is prepared by hydrolysis of anaqueous solution of ceric nitrate in a nitric acid medium, thenseparation of the precipitate obtained, washing with an organic solvent,possibly drying, and then calcination. The ceric oxide obtained has aninteresting specific surface when it is prepared in a calcinationtemperature range of from 300 to 600° C. However, a drop in specificsurface is noted after calcination at a higher temperature, with thespecific surface being 10 m² /g after calcination at 800° C.

FR-A 2,559,755 can also be cited, which relates to a ceric oxide havinga specific surface of at least 85±5 m² /g after calcination between 350and 500° C. and, preferably, between 150 and 180 m² /g after calcinationbetween 400 and 450° C. This oxide is obtained using a method whichconsists of precipitating a base ceric sulfate by reacting an aqueoussolution of ceric nitrate and an aqueous solution containing sulfateions, of separating the precipitate obtained, washing it using anammonia solution, possibly of drying it and then calcining it at atemperature varying between 300 and 500° C. The ceric oxide prepared inthis manner has a large specific surface, but when it is subjected to acalcination operation at 800° C., its specific surface decreasesconsiderably and is about 10 m² /g.

One of the objects of the present invention is to provide a ceric oxidewhich has a large specific surface at high temperatures.

Another object of the invention is a method enabling such an oxide to beobtained.

The characteristic of the ceric oxide of the invention is that it has aspecific surface of at least 15 m² /g measured after calcination at atemperature of between 800 and 900° C.

The preferred ceric oxide of the invention has a specific surface ofbetween 20 and 60 m² /g measured after calcination at a temperature of800° C.

In accordance with the calcination conditions of ceric hydroxide, cericoxide has a specific surface between 15 and 160 m² /g measured aftercalcination at a temperature varying between 350 and 900° C.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph on which is shown the curve (A) of variation of thespecific surface (expressed in m² /g) of a ceric oxide of the inventionas a function of the calcination temperature given in ° C.

The ceric oxide of the invention has a specific surface of at least 15m² /g, measured after calcination at a temperature of between 800° C.and 900° C., and a specific surface greater than said surface aftercalcination at a temperature below the above-indicated range.

Thus, it can have a specific surface varying between 70 and 160 m² /gand, preferably, between 100 and 160 m² /g, measured after calcinationbetween 350 and 450° C. However, when it is subjected to a highertemperature of up to 900° C., at the time of its use, particularly inthe catalysis field, it has the characteristic of retaining a specificsurface of at least 15 m² /g and, preferably, between 20 and 60 m² /gwhen it is subjected to a temperature of 800° C.

In the present application, the specific surfaces expressed are measuredon a product having undergone calcination for at least 2 hours.

Another characteristic of the ceric oxide of the invention is that ithas a porous volume of greater than 0.1 cm³ /g at a measurementtemperature of between 800 and 900° C. and, preferably, greater than0.15 cm³ /g.

The porous volume, which corresponds to pores with a diameter of lessthan 60 nm (600 Å), is measured with a mercury porosimeter in accordancewith the ASTM D4284-83 standard or using the isotherm nitrogenadsorption method--the above-identified B.E.T. method.

Like the specific surface, the porous volume depends on the calcinationtemperature: it can vary between 0.35 and 0.15 cm³ /g for a calcinationtemperature of between 350 and 900° C.

The preferred ceric oxide of the invention has a porous volume ofbetween 0.15 and 0.25 cm³ /g after calcination at a temperature of 800°C.

The size of the pores of a ceric oxide calcined at 800° C. rangesbetween 3 nm (30 Å) and 60 nm (600 Å); the average diameter (d₅₀) of thepores varies between 20 nm (200 Å) and 30 nm (300 Å) and is preferablyapproximately 25 nm (250 Å).

The average diameter is defined as being a diameter such that all thepores smaller than said diameter constitute 50% of the total porousvolume (Vp) of pores with a diameter less than 60 nm (600 Å).

A ceric oxide calcined at 350° C. has pores of from 2 nm (20 Å) to 100nm (1,000 Å), with the average diameter varying form 10 nm (100 Å) to 20nm (200 Å) and, preferably, about 15 nm (150 Å).

Analysis using X-ray diffraction shows that the ceric oxide of theinvention has a crystalline phase of the CeO₂ type with a mesh parametervarying from 0.542 nm (5.42 Å) to 0.544 nm (5.44 Å). As an indication,it should be pointed out that the size of the crystallites of a cericoxide obtained after calcination at 350° C. is between 4 nm (40 Å) and 6nm (60 Å) and after calcination at 800° C. is between 10 nm (100 Å) and20 nm (200 Å).

The method for obtaining the ceric oxide having a specific surface of atleast 15 m² /g for a measurement temperature of between 800° C. and 900°C. is characterized by the fact that it consists:

of preparing a ceric hydroxide by reacting a solution of cerium salt anda base, possibly in the presence of an oxidizing agent, with the amountof the base being such that the pH of the reaction medium is greaterthan 7; of separating the precipitate obtained, and possibly washing it;

of placing the ceric hydroxide in suspension in water or in an aqueoussolution of a decomposable base;

of heating it in a closed chamber to a temperature and a pressurerespectively lower than the critical temperature and the criticalpressure of said medium;

of cooling the reaction mixture and bringing it to atmospheric pressure;

of separating the ceric hydroxide treated in this manner;

then of calcining it.

Applicant has found that a ceric oxide with a large specific surface athigh temperature could be obtained by subjecting a ceric hydroxide orceric oxide hydrate obtained by base precipitation using a solution of acerium salt under well-defined conditions, with an autoclave treatmentcarried out in water or in an aqueous solution of a decomposable base,before the calcination operation.

A ceric hydroxide is used in the method of the invention which isprepared in accordance with the method described below which consists ofreacting a cerium salt solution and a base, possibly in the presence ofan oxidizing agent and separating the precipitate obtained, possibly ofwashing it, and/or drying it.

The cerium salt solution used can be any aqueous cerium salt solution inthe cerous and/or ceric state which is soluble in the conditions ofpreparation, in particular a cerous chloride or cerium nitrate solutionin the cerous or ceric state of a mixture of same.

The cerium salt solution is selected such that it contains no impuritieswhich can be found in the calcined product. It can be advantageous touse a cerium salt with a degree of purity of above 99%.

The concentration of the cerium salt solution is not a critical factor,in accordance with the invention, and it can vary within wide limits; aconcentration of between 0.2 and 4 moles per liter is preferred.

In accordance with a preferred embodiment, the cerium is introduced intothe reaction medium in the cerous state and is oxidized to the cericstate using an oxidizing agent.

Among the oxidizing agents which are suitable are, in particular,solutions of sodium, potassium or ammonium perchlorate, chlorate,hypochlorite, or persulfate, hydrogen peroxide or air, oxygen or ozone.Hydrogen peroxide is preferably used.

The amount of oxidizing agent in relation to the cerous salt to beoxidized can vary within wide limits. It is generally greater than thestoichiometry and preferably corresponds to an excess of between 10 and40%.

Another preferred embodiment of the invention consists of using anaqueous solution of ceric nitrate. Ceric nitrate generally has a certaininitial acidity and can have a normality varying between 0.01N and 5N.The concentration of H⁺ ions is not critical. It is desirable for it tobe between 0.1N and 1N.

By way of starting materials, a ceric nitrate solution can be used whichis obtained by the action of nitric acid on a hydrated ceric oxideprepared in a conventional manner, for example, by the action of nitricacid on the cerous carbonate and addition of an ammonia solution in thepresence of an oxidizing agent, preferably hydrogen peroxide.

The ceric nitrate solution obtained in accordance with the method ofelectrolytic oxidation of a cerous nitrate solution, which is describedin French Patent Application FR-A 2,570,087 (No. 84 13541), is an idealstarting material.

The precipitation of the ceric hydroxide can be carried out by reactionof the cerium salt solution and a base solution.

The base solution used can, in particular, be an aqueous solution ofammonia or of sodium or potassium hydroxide. An ammonia solution ispreferably used. The normality of the base solution used is not acritical factor in accordance with the invention; it can vary withinwide limits and will, however, be preferably between 1 and 5N,preferably 2 to 3N.

The quantity of base added is determined such that the pH of thereaction medium is greater than 7. It is preferably greater than 7.0 andless than approximately 10 and even more preferably between 7.5 and 9.0.

It can be particularly advantageous to adjust the pH within these limitsto a constant value at ±0.1 units of pH.

The temperature of the reaction medium should preferably be between 5and 70° C. and, more especially, between 40 and 70° C. The mixing timein the reaction medium is not a critical factor in accordance with theinvention and can vary within wide limits; generally times of between 15minutes and 2 hours are selected.

A precipitate is obtained which can be separated in accordance withconventional solid/liquid separation techniques, such as decanting,drying, filtration and/or centrifuging.

It is desirable to wash the separated precipitate so as remove theremaining anions adsorbed on the precipitate.

The washing is preferably carried out using water or a base solution,preferably with a concentration of between 1 and 5N. An ammonia solutionis preferably sued. One or several washings can be carried out, mostfrequently from one to three washings.

The separated, and preferably washed, ceric hydroxide can be useddirectly in the method of the invention. It is also possible to use aceric hydroxide having undergone a drying operation. The drying can becarried out in air or under reduced pressure, for example on the orderof 1 to 100 mm of mercury (133.322 Pa to 13332.2 Pa). The dryingtemperature can vary between room temperature and 100° C. and the dryingtime is not critical and can be between 2 to 48 hours. The dryingoperation is optional and too much drying is not desirable.

The ceric hydroxide prepared in accordance with the above-describedmethod of precipitation and preferably used corresponds to the followingformula (I):

    Ce(OH).sub.x (X).sub.y,n H.sub.2 O                         (I)

wherein:

X represents a chloride or nitrate anion,

y is less than 0.5,

x=4-y,

n varies between 0 and approximately 20.

The preferred compound corresponds to formula (I) in which y is between0 and 0.1 Even more preferably, X represents a nitrate anion.

The preferred compound corresponding to formula (I) is prepared byreacting a solution of cerous chloride or cerous nitrate and a solutionof ammonia, in the presence of hydrogen peroxide, of separating theprecipitate obtained and submitting it to at least one washing,preferably with water.

The amount of base added is such that the reaction pH is greater than 7and, preferably, between 7.5 and 9.

The temperature of the reaction medium is selected between 5 and 70° C.,preferably between 40 and 70° C.

The ceric hydroxide obtained in accordance with the method describedabove is preferably used in the method which is the object of thepresent invention.

In accordance with the method of the invention, the ceric hydroxide isused in the form of a suspension in water or in an aqueous solution of abase which is decomposable in the calcination conditions of theinvention.

"Decomposable base" is used to mean a compound with a pH of less than 7which is capable of being decomposed under the calcination conditions ofthe invention.

The following can be cited as illustrative of such bases: urea, ammoniumacetate, ammonium hydrogenocarbonate, ammonium carbonate, or a primary,secondary or tertiary amine, such as, for example, methylamine,ethylamine, propylamine, n-butylamine, sec-butylamine, n-pentylamine,2-amino pentane, 2-amino 2-methyl butane, 1-amino 3-methyl butane,1,2-diamino ethane, 1,2-diamino propane, 1,3-diamino propane,1,4-diamino butane, 1,5-diamino pentane, 1,6-diamino hexane,dimethylamine, diethylamine, trimethylamine, triethylamine, or aquaternary amine such as, for example, a tetraalkylammonium hydroxidepreferably with alkyl radicals containing from 1 to 4 carbon atoms, andmore particularly tetramethylammonium hydroxide or tetraethylammoniumhydroxide is used.

A mixture of bases can also be used.

A preferred embodiment of the method of the invention consists of usinga solution of a decomposable base since it has unexpectedly been foundthat the fact of carrying out the autoclave operation in such a mediumenables not only the specific surface of the ceric oxide obtained to beincreased, but also provides a greater specific and porous volume up totemperatures of 900° C.

Preferably a solution of ammonia, tetraalkylammonium hydroxide ormixtures thereof are used.

When the liquid medium is a base solution, the concentration thereof isnot a critical factor in accordance with the invention. It can varywithin wide limits, for example, between 0.1 and 11N, but it ispreferable to use solutions whose concentration varies between 1 and10N.

In the liquid medium the concentration of ceric hydroxide expressed inCeO₂ can vary between 0.3 and 6 moles/liter, and preferably between 2and 3 moles/liter.

The autoclave operation is carried out a temperature situated betweenthe reflux temperature and the critical temperature of the reactionmedium. Preferably, a temperature of between 100° C. and 350° C. isselected, and even more preferably, between 150° C. and 350° C.

The rise in temperature takes place at a speed which is not critical.The reaction is reached by heating, for example, for between 30 minutesand 4 hours.

The method of the invention can be carried out by placing the cerichydroxide in suspension in the liquid medium into a closed chamber. Thepressure therefore only results from the heating of the reactionmixture.

Under the temperature conditions given above and in an aqueous medium,it can be pointed out, by way of indication, that the pressure variesbetween 1(10⁵ Pa) and 165 Bars (165.10⁵ Pa), preferably between 5(5. 10⁵Pa) and 165 Bars (165.10⁵ Pa).

It is also possible to exert outside pressure which then adds to thatresulting from the heating.

The length of the autoclave operation is not critical. It can varybetween 30 minutes and 6 hours.

At the end thereof, it is left to cool using the inertia of the systemand the system is returned to atmospheric pressure.

The product in suspension in the liquid medium is separated usingconventional solid-liquid separation techniques, such as decanting,drying, filtration and/or centrifuging.

The collected product can possibly be washed and/or dried under theconditions previously described.

In accordance with a last step of the method of the invention, theproduct obtained is calcined at a temperature of between 300° C. and1,000° C. and, preferably, between 350° C. and 800° C.

The calcination time can vary within wide limits between 30 minutes and10 hours, preferably between 2 and 6 hours.

The ceric oxide in accordance with the invention has a large specificsurface at high temperatures such that it is fully suitable for thecatalysis field, as a catalyst or as a catalytic support.

It is particularly appropriate for use as a catalytic support inreactions for the treatment of exhaust gases of internal combustionmotors.

The examples which follow illustrate the invention without, however,limiting it.

Examples 1 to 9 relate to the new ceric oxide of the invention and themethod for obtaining same.

Tests A and B are given by way of comparison; they do not include anautoclave treatment.

EXAMPLE 1 Test A 1. Synthesis of the Ceric Hydroxide

922 cm³ of a solution of cerous nitrate containing 179 g/l of CeO₂ and38 cm³ of a solution of 200 volume hydrogen peroxide at 50° C. wereplaced in a reactor with a double casing having a useful volume of 2liters and equipped with a stirring device and a system for introductionof the reagent (doser pump).

825 cm³ of an aqueous solution of 4N ammonia was added until a pH of 8.4was obtained.

With the reagents added, the reaction medium was maintained at 70° C.for 1 hour.

The precipitate was then separated on a Buchner.

569 g of a ceric hydroxide with a NO₃ /Ce molar ratio of 0.1 wereobtained.

X-ray diffraction analysis showed that the ceric hydroxide hadcrystallites with a diameter of 3 nm (30 Å).

2. Autoclaving the Ceric Hydroxide

30 cm³ of deionized water and 30 g of the ceric hydroxide prepared abovewere placed successively in a tall 100 cm³ beaker.

After homogenization of said ceric hydroxide in its medium, the beakerwas placed in an autoclave having a useful volume of approximately 0.51.

This was brought to 200° C., which is approximately 16 bars (16.10⁵ Pa)for 4 hours using suitable heating means.

At the end of this hydrothermal treatment, the precipitate was filteredon a Buchner.

Using X-ray diffraction on the moist product, an average crystallitediameter of 4.5 nm (45 Å) was determined.

The product then underwent a calcination operation for 2 hours, with onefraction being calcined at 350° C. and the other at 800° C.

Using the methods defined in the description, the specific surface ofthe ceric oxide obtained and its porous volume <60 nm (<60 Å) were thendetermined.

X-ray diffraction was also carried out to determine the size of thecrystallites perpendicular to directions 110 and 220.

The results obtained are given in Table I. For purposes of comparison,the results obtained with a ceric oxide prepared using directcalcination at 350° C. and at 800° C. for 2 hours from the cerichydroxide synthesized under 1 (Test A) are indicated.

                  TABLE I                                                         ______________________________________                                                                          Diameter of                                         Calcination                                                                            Specific Porous  the crys-                                           temperature                                                                            surface  volume  tallites                                            °C.                                                                             m.sup.2 /g                                                                             cm.sup.3 /g                                                                           nm (Å                                   ______________________________________                                        Example 1 350        110      0.20  5.0   (50)                                          800        21       0.08  --                                        Test A    350        46       0.16  6.5   (65)                                (without  800        11       0.06  30    (300)                               autoclaving)                                                                  ______________________________________                                    

When the ceric oxide is calcined at a higher temperature, a decrease insurface and in the porous volume are noted due to the considerablefritting which translates into the high increase of the crystallitesbetween 350° C. and 800° C.

However, the favorable effect of the autoclaving treatment is noted onthe specific surface and the porous volume.

EXAMPLE 2 Test A 1. Synthesis of the Ceric Hydroxide

This was carried out using the method of Example 1--1.

2. Autoclaving of the Ceric Hydroxide

Using the operating protocol described in Example 1, 30 g of the cerichydroxide prepared above, placed in suspension in 30 cm³ of an aqueous1N ammonia solution, were subjected to treatment in an autoclave at 200°C. for 4 hours.

Using X-ray diffraction on the moist product, an average crystallitediameter of 4 nm (40 Å) was determined.

At the end of this hydrothermal treatment, the precipitate was filteredon a Buchner.

This was then subjected to a calcination operation for 2 hours: with onefraction being calcined at 350° C. and the other at 800° C.

The specific surface, the porous volume and the size of the crystallitesof the ceric oxide having undergone the autoclaving treatment (Example2) were then determined and, for purposes of comparison, those of theceric oxide prepared by direct calcination at 350° C. and at 800° C. for2 hours from the ceric hydroxide synthesized under 1 (Test A).

The results obtained are given in Table II.

                  TABLE II                                                        ______________________________________                                                                          Diameter of                                         Calcination                                                                            Specific Porous  the crys-                                           temperature                                                                            surface  volume  tallites                                            °C.                                                                             m.sup.2 /g                                                                             cm.sup.3 /g                                                                           nm (Å                                   ______________________________________                                        Example 2 350        127      0.35  4.5   (45)                                          800        30       0.19  19    (190)                               Test A    350        46       0.16  6.5   (65)                                (without  800        11       0.06  30    (300)                               autoclaving                                                                   ______________________________________                                    

As in the preceding example, the favorable effect of the autoclaving onthe specific surface and the porous volume of the ceric oxide obtainedafter calcination at 800° C. are observed.

EXAMPLE 3 1. Synthesis of the Ceric Hydroxide

This was carried out using the method used in Example 1--1.

2. Autoclaving of the Ceric Hydroxide

25 cm³ of an aqueous solution of 20% tetraethylammonium hydroxide and 15g of the ceric hydroxide prepared above were placed successively in atall 100 cm³ beaker.

After homogenization of the medium, the beaker was placed in theautoclave.

This was brought to 200° C., which is approximately 16 bars (16.10⁵ Pa)for 3 hours using suitable heating means.

At the end of this hydrothermal treatment, the precipitate was filteredon a Buchner.

The product then underwent a calcination operation under the followingconditions: one fraction was calcined for 2 hours at 350° C. and theother was calcined for 1 hour at 800° C.

The specific surface and the porous volume of the ceric oxides obtainedwere then determined.

The results obtained are given in Table III.

                  TABLE III                                                       ______________________________________                                                Calcination  Specific Porous                                                  temperature  surface  volume                                                  °C.   m.sup.2 /g                                                                             cm.sup.3 /g                                     ______________________________________                                        Example 3 350            115      0.53                                                  800             29      0.23                                        ______________________________________                                    

EXAMPLES 4 TO 9 Test B 1. Synthesis of the Ceric Hydroxide

922 cm³ of a solution of cerous nitrate containing 150 g/l of CeO₂ and38 cm³ of a solution of 200 volume hydrogen peroxide were placed at roomtemperature into an apparatus as described in Example 1.

150 cm³ of an aqueous 3N ammonia solution were added, while maintainingthe temperature at 8° C., until a pH equal to 9.5 was obtained.

With the reagents added, the reaction medium was maintained at 8° C. for1 hour.

The separation of the precipitate was then carried out on a Buchner andwashed with water.

Analysis using X-ray diffraction showed that the ceric hydroxide hadcrystallites with an average diameter of 3.5 nm (35 Å).

2. Autoclaving of the Ceric Hydroxide

A series of tests was carried out in which the autoclaving temperaturewas varied between 160° and 330° C.

Using the same operating protocol as described in the precedingexamples, 150 g of ceric hydroxide prepared above were placed insuspension in 150 cm³ of an aqueous 1N ammonia solution and subjected totreatment in the autoclave for 4 hours.

At the end of this heat treatment, the precipitate was filtered on aBuchner.

This was then subjected to a calcination operation under the followingconditions: one fraction was calcined for 2 hours at 350° C. and theother calcined for 2 hours at 800° C.

The specific surface and the porous volume of the ceric oxides obtainedwere then determined.

For purposes of comparison, the results are given which were obtainedwith a ceric oxide prepared by direct calcination at 350° C. and at 800°C. for 2 hours from the ceric hydroxide synthesized under 1 (Test B).

All the results obtained are given in Table IV.

                  TABLE IV                                                        ______________________________________                                               Autoclaving                                                                            Calcination                                                                              Specific  Porous                                          temperature                                                                            temperature                                                                              surface   volume                                          °C.                                                                             °C. m.sup.2 /g                                                                              cm.sup.3 /g                              ______________________________________                                        Example 4                                                                              160        350        69                                                      160        800        20                                             Example 5                                                                              180        350        85                                                      180        800        20                                             Example 6                                                                              200        350        131     0.24                                            200        800        26      0.24                                   Example 7                                                                              250        350        126     0.27                                            250        800        27      0.14                                   Example 8                                                                              300        350        81      0.20                                            300        800        31      0.12                                   Example 9                                                                              330        350        73      0.25                                            330        800        45      0.17                                   Test B   --         350        65      0.12                                                       800        9.7     0.07                                   ______________________________________                                    

It can be noted that the ceric oxides of the invention have highspecific surfaces measured after calcination at 800° C.

EXAMPLE 10 Test C 1. Synthesis of the Ceric Hydroxide

Into a reactor with a double casing in which heat-regulated watercirculated at 20° C. and with a useful capacity of 2,000 cm³, equippedwith a stirring device and a system for introduction of the reagents,the following were simultaneously and continuously placed:

an aqueous solution of ceric nitrate containing 1 mole/liter of ceriumIV, 0.06 mole/liter of cerium III and with a free acidity of 0.5N,prepared by electrolysis in accordance with FR-A 2,570,087 (No. 8413641), at a rate of 0.78 liter/hour,

an aqueous solution of 3N ammonia, at a rate of 1.22 liters/hour.

Mixing was carried out under stirring at 300 rpm for 60 minutes.

The rates of addition of the starting solutions were regulated such thatthe pH was maintained at 9.0.

The precipitate formed was separated on a Buchner.

A product containing 20% by weight of ceric oxide and with a crystallitesize of less than 3 nm (30 Å) was obtained.

2. Autoclaving of the Ceric Hydroxide

In an autoclave having a useful volume of 1.5 1, 300 cm³ of NH₄ OH 1Nand 100 g of the ceric hydroxide prepared above were placedsuccessively.

After homogenization of said ceric hydroxide in its medium, this wasbrought to 200° C., which is approximately 16 bars (16.10⁵ Pa) for 3hours using suitable heating means.

At the end of this hydrothermal treatment, the precipitate was filteredon a Buchner.

Two fractions of this moist product then underwent a calcinationoperation under the following conditions: 2 hours at 350° C. and 1 hourat 800° C.

Using the methods defined in the description, the specific surface ofthe ceric oxide obtained and its porous volume were then determined.

The results obtained are given in Table V.

For purposes of comparison, the results obtained with a ceric oxideprepared using direct calcination at 350° C. for 2 hours and at 800° C.for 1 hour from the ceric hydroxide synthesized under 1 (Test A) areindicated.

                  TABLE V                                                         ______________________________________                                                 Calcination Specific Porous                                                   temperature surface  volume                                                   °C.  m.sup.2 /g                                                                             cm.sup.3 /g                                     ______________________________________                                        Example 10 350           155      0.24                                                   800           17       0.06                                        Test C     350           53       0.05                                        (without   800           4.9      0.03                                        autoclaving)                                                                  ______________________________________                                    

The favor able effect of the autoclaving can be noted on the specificsurface and the porous volume of the ceric oxide after calcination at800° C.

I claim:
 1. A ceric oxide which has a specific surface of at least 15 m²/g, measured after calcination at a temperature of between 800 and 900°C.
 2. The ceric oxide in accordance with claim 1, which has a specificsurface of between 20 and 60 m² /g, measured after calcination at atemperature of 800° C.
 3. The ceric oxide in accordance with one ofclaims 1 and 2, which has a porous volume of at least 0.1 cm³ /gmeasured after calcination at a temperature of between 800° C. and 900°C.
 4. The ceric oxide in accordance with claim 3, which has a porousvolume of between 0.15 and 0.25 cm² /g, after calcination at atemperature of 800° C.
 5. The ceric oxide in accordance with one ofclaims 1 to 4, which has an average pore diameter varying between 20 nm(200 Å) and 30 nm (300 Å), after calcination at a temperature of 800° C.6. The ceric oxide in accordance with one of claims 1 and 2, which has aspecific surface of between 15 and 160 m² /g, measured after calcinationat a temperature of between 350° C. and 900° C.
 7. The ceric oxide inaccordance with claim 6, which has a porous volume varying from 0.35 and0.15 cm³ /g after calcination at a temperature varying from 350° C. to900° C.
 8. A ceric oxide, which has a specific surface of between 70 and160 m² /g, measured after calcination at a temperature of between 350°C. and 450° C., and that it retains a specific surface of at least 15 m²/g when it is subjected to a temperature of between 800° C. and 900° C.9. The ceric oxide in accordance with claim 8, which retains a specificsurface of between 20 and 60 m² /g, when it is subjected to atemperature of 800° C.
 10. The ceric oxide in accordance with one ofclaims 8 and 9, which has a specific surface of between 100 and 160 m²/g, measured after calcination at a temperature of between 350° C. and450° C.
 11. The ceric oxide in accordance with one of claims 8 and 9,which has an average pore diameter varying between 10 nm (100 Å) and 20nm (200 Å), after calcination at a temperature of 350° C.
 12. The cericoxide in accordance with claim 10 which has an average particle diametervarying between 10 nm (100 Å) and 20 nm (200 Å), after calcination at atemperature of 350° C.